This invention generally relates to apparatus for perforating sheet material, and more particularly to a rotary punch having a spherical punch mounted on a rotating punch carrier and a die mounted on a die holder which rotates in a direction opposite the punch carrier.
Rotary punches are used to make a series of holes in continuous sheet material. For instance, the holes adjacent the side margins of tractor feed computer printer paper are frequently made using rotary punches. This type of punch accurately and inexpensively makes holes in sheet material at material feed speeds up to 600 feet per minute or more.
Many prior art rotary punches have cylindrical pin punches. The leading and trailing corners of the pin punches contact the corners of the dies as each hole is made because the punches and dies rotate relative to each other. This contact causes the punches and dies to wear, eventually requiring they be replaced. The contact also causes heat which can damage some sheet materials or cause the materials to melt, thereby gumming up the punches and dies and reducing their ability to perforate the sheet material. Moreover, the heat can embrittle the punches, increasing their potential to break.
In order to alleviate these problems, some prior art rotary punches use spherical punches which do not make as much contact with the dies as pin punches. Thus, the heat generated by spherical punches is less than that generated by pin punches, so the problems associated with heat buildup are reduced. The spherical punches are biased against the dies by springs in many of the prior art rotary punches. However, spring-loaded punches require lengthy punch spacings to accommodate the large springs required to cut tough materials. These punch spacings are unacceptably large for some applications.